Crystal oscillator monitor and centralized control



Apri 6, 1937. H. H. BEVERAGE 2,076,351

CRYSTAL OSCILLATOR MONITOR AND CENTRALIZED CONTROL Filed April 21, 19335 Sheets-Sheetl 1 t E n.

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lApril 6, 1937@ H. H. BEVERAGE CRYSTAL OSCILLATOR MONITOR ANDCENTRALIZED CONTROL Filed April 21, 1953 5 sheets-Sheet 2 April 6, 1937.H. H. BEVERAGE 2,076,361

CRYSTAL OSCILLATOR MONITOR AND CENTRALIZED CONTROL Filed April 2l, 19353 Sheets-Sheet 3 INVENTOR- H H BEVERAGE ATTORNEY- Patented Apr. 6, 1937PATENT OFFICE CRYSTAL OSCILLATOR MONITOR AND CENTRALIZED CONTROL HaroldH. Beverage, Riverhead, N. Y., assig'nor to Radio Corporation oi'America., a corporation of Delaware Application April 21, 1933, SerialNo. 667,253

10 Claims.

The present invention relates to a novel method of and circuitarrangement for producing oscillations by means of a quartz crystaloscillator, and utilizing said oscillations as a frequency standard t9kproduce harmonics as markers throughout fthe radio frequency spectrumcovering the different frequencies used in the signaling system. Thestandard frequencies and the harmonics thereof are utilized inaccordance with the present method and circuit for checking thefrequency of any or all of the transmitters of the system and forproducing indications of the amount by which the transmitters havedeviated from their assigned frequency. This determining of thefrequency of the transmitters is accomplished by a novel and simplifiedmethod and circuit which eliminates the necessity of interpolationsbetween harmonics.

The invention also involves a method of and circuit for controlling froma remote point any of the transmitters of the system. The object of theremote control is to eliminate the necessity of attendance at some ofthe transmitters. By means of the method of remote control and circuittherefor of the present invention the circuit at the remote control maybe turned on or off, the attendant at the transmitter may be called, orany central station may send ring calls over the control circuits to anyother central station.

A particular feature of the present invention is a novel method of andcircuit for tuning any or all of the transmitters of the system back totheir assigned frequency if my method of and circuit for checking thefrequency of said trans- 35 mitters, as described in the secondpreceding paragraph, shows that such transmitter or transmitters are oitheir assigned frequency.

In connection with the ultra short wave interisland telephone systemwhich the Mutual |I'ele- 4g phone Company has installed in the HawaiianIslands, I have suggested certain equipment and certain ideas for thepurpose of facilitating the adjustment and the maintenance of the entireradiotelephone network from a lcommon station.

4;, The Mutual Telephone Company was anxious to obtain a system whichcould be left relatively unattended at the outlying islands; theydesired means for quickly checking the frequency of all of thetransmitters in the system to see that they 50 were'on their properfrequencies.

I proposed obtaining frequencyV assignments from the Federal RadioCommission which corresponded with the harmonics of a single quartzcrystal oscillator. By the use of such a device,

55 the harmonics of the oscillatorwould light ex- (Cl. Z50-17) actly onthe assigned frequencies of the transmitters and if the transmitterswere on frequency. they should beat with the proper crystal harmonic atthe control station. If it should then be found that the transmitter didnot beat with the crystal harmonic, it would be an indication that thetransmitter was off frequency and it would then be desirable to obtainan approximate measure of how much the transmitter was off frequency andto further have some meansy circuit by modulating one of the localtransmitters. One of these tones will be used to operate the dialselector at the distant point, while the second tone would be used tooperate a simple on-off relay after the proper point had been selectedon the dial, thereby operating a motor, for example, which would tunethe transmitter back on to its proper frequency.

In the prior art, it has been customary to use a quartz crystaloscillator as a frequency standard and to use the harmonics of thisoscillator as markers throughout the radio frequency spectrum over whichit was desired to make measurements. A comparatively complicatedarrangement was required to interpolate between these harmonics in orderto accurately determine the frequency of a station which lay between aparticular pair of harmonics. A universal method of making thisinterpolation has been described in Peterson United States applicationSerial No. 535,910, filed May 8, 1931. In the present lnvention, it isnot necessary to interpolate between harmonics and the only requirementis, that a simple means be provided for approximately determining asmall frequency change close to one of the harmonics. A means foraccomplishing this result is quite simple and will be described in moredetail later. i

In regard to the second feature of this invention, that is, the remotecontrol idea, it has been the usual practice to have an attendant ateach yof the transmitting stations to maintain the transmitter in properadjusment on its assigned frequency. By means of the invention describedherewith, continuous attendance at the transmitter is not required sincethe adjustments which must be most frequently made, can be made from aremote control center.

The novel features of my invention as required by law have been pointedvout with particularlty in the claims appended hereto.

'Ihe method of determining the frequencies and of controlling thestations of the system from any one of aplurality of remote points, andcircuits for carrying out the same, will be understood by the followingdetailed description thereof and therefrom when read in connection withthe attached drawings, in which:

Figure 1 shows diagrammatically a control station including the regulartraflic receiver, a ,transmitter connected therewith, and the frequencychecking receiver and crystal oscillator which forms a feature of thepresent invention;

Figure 2 shows a controlled stationat-a remote point; while,

Figure 3 shows a modification of the prior arrangements.

In Figure 1, I have shown the equipment required at the central controlstation. A monitor receiver including an aerial I8, radio frequencyamplifier I 9, demodulator 2i)v of vthe heterodyne type, oscillator O,intermediate frequency amplifier 2|, etc., is shown. 'I'he receiver maybe tuned to the frequency of any of the transmitters in theradiotelephone network.` The transmitters in this network are set upwith a uniform frequency spacing, such that their frequencies correspondwith the harmonics of a crystal oscillator 26. 'I'he outputof thecrystaloscillator is amplified and applied to a negatively biased tube intheharmonic generator 21. I need not describe this harmonic generator indetail, as it has already been described in United States applicationNo. 535,910, filed May 8, 1931, referred to above.

The harmonics from the harmonic generator 21 are applied to the highfrequency detector or rst detector, of the monitor receiver 20. Now ifthe monitor receiver I8, I9, etc., is tuned to one of the transmittersof thenetwork, the trans- :hitter-frequencyv and the crystal harmonicfrequency corresponding to this transmitter frequency should both bepractically the same and both are applied to the high frequency detector20, where they are combined to produce a lower frequency, i. e., thediiference frequency, and are amplified by the intermediate frequencyamplifier 2|, which may, for example, be tuned to 1000 kilocycles. Ifthe transmitter frequency and the crystal harmonic are close together,they will be detected in the intermediate frequency detector 22, andwill be amplified by audio frequency amplifier 23, and the beat notebetween them will be audible `in telephone 24. In lthe statement' ofoperation given above it is assumed that the transmitter is on afrequency suiliciently close to its assigned frequency to produce anaudible beat note in the telephone 24. If, however, the transmitter hasdrifted off frequency more than 10 or 15 kilocycles, no beat will beheard in telephone 24, and it will be necessary to put oscillator 25into operation. This oscillator is calibrated over a range of say 900 to1100 kilocycles. If the oscillator 25 is set at 1000 kilocycles, and theoscillations in the output of the high frequency detector 20 areadjusted until the proper harmonic from crystal oscillator 26 is heardat zero beat in telephone 24, then a simple.

tween the crystal harmonic and the tranSmitter I' frequency. 'I'hisadjustment merely consists in raising or lowering the frequency ofoscillator 25 until it is at zero beat with .theintermediate frequencyproduced in the receiver intermediate frequency amplifier by thetransmitter. If, for example, the transmitter zero beat occurred, onsetting 25, at 980 kilocycles, then we would know that the transmitterwas 1000 minus 980,'or 20 kilocycles below its assigned frequency. Contoa frequency F (I9 is not operating) and then introduces into 20 theharmonic of crystal 26 which is equal to this frequency F.v The highfrequency oscillator O is now adjusted to about a frequency X which,when beat with the frequencyF, produces a beat note of frequency Y towhich the intermediate frequency amplifier 2| is tuned. Now, if thecalibrated oscillator 25 is turned on and set at a frequency Y, a zeroybeat note should be produced in 23. If the beat note is not zerofrequency it indicates that the high frequency oscillator O is not tunedexactly to the frequency X. 'I'he operator accordingly tunes O until thebeat note frequency is reduced to zero. The tune of O is now knownbecause the oscillator 25 is calibrated and the harmonic F of 26 isknown. Now the radio frequency amplifier I9 is turned on and tuned tothe wave of the transmitter to be checked whose frequency is F+, butshould be F. This wave of a frequency F+ -beats with the wave of afrequency X produced in O to produce in 2| a. wave of frequency Y+.'Ihis wave beats with Y (say 1000 kc. wave) produced by 25 and a beatnote equal to Y+ Y is heard in the phones. 25 is now adjusted until zerobeat is produced and the setting of 25 shows the kilocycle deviation ofthe transmitter from its assigned harmonic. The oscillator 26 is anaccurate precision oscillator. The oscillator 25 measures differencefrequencies.

T'he device described above permits the operator at the control stationto ascertain in a simple and novel manner whether the transmitters ofthe system are on their assigned frequencies.

If a transmitter has shifted from its assigned frequency it may bebrought back into tune by the novel device described hereinafter.

In Figure 1, I have also indicated the incoming line from the centraloflice terminating inthe hybrid coil I, with a balancing network 3. The

incoming signal from one of the distant stations would be received onantenna I1 and receiver I6 and applied to the midpoint of thehybrid coilwhere the energy divides between the line to the central oillce and thenetwork. AThe output from` the receiver then divides equally between theline and the network, so that no energy is induced in winding 2. yWinding 2 is the output of the hybrid coil and carries the voice fromthe line over to the transmitter which is represented as including theessential elements including modulator 6 and power amplifier 1. Thevoice frequency currents are passed through a low pass lter 4 andcoupling'tube 5, for reasons which will be described later. There arealso two oscillators I3 and I4, the oscillations from which may beapplied to the modulator 6 .for the purpose of controlling the controlstation by means of the dial.

circuits at the remote station. The oscillator I4 produces a tone whichis utilized at the remote station to actuate a. circuit selector relay.'I'he oscillator I3 produces a tone which is used at the remote stationto actuate a control relay. The invention is not limited to thefrequencies selected since, obviously, other frequencies may be used.

If the dial 5 is operated it keys the 2700 cycles oscillator |4 andsends this modulated interrupted tone out over the transmitter 1. At theremote station shown in Figure 2 this modulated signal is picked up onantenna 28 and receiver 29 and the 2700 cycle tone is selected by bandpass filter 3|, and is further amplified and rectified at 32, and isapplied to a quick acting selector control relay 33. The contacts 34 cithis relay 33 are held open by a spring S but the relay will close inaccordance with the impulses sent out from Closing the contact 34 ofthis relay connects two relays 35 and 36 with the energy source B. Thusthe relay 33 actuates the two other relays 35 and 36. Relay 36 is aholding relay which actuates the holding pawl 31 and its operation ismade slow by means of a dashpot DP. Relay 35 is rapid acting. When animpulse at 2700 cycles comes in the relay 33 closes its contact 34 inresponse to said impulse resulting-from operation of dial l5. Relay 35is energized and operates and pulls the drag 38 and moves the ratchetwheel RW in a clockwise direction one notch. The ratchet wheel RW isthen held in position by the latch or pawl 31. If a second impulse of2700 cycles is then received, the ratchet wheel switch is notched up onemore point. It is obvious that the ratchet wheel switch will be advanceda number of points corresponding with the number of impulses at 2700cycles sent over the circuit by the dial I5 at the control station, and,that the switch 39 will be held stationary on the final notch by thelatch or pawl 31.

Now if the oscillator I3 of Figure 1 is energized by closing switch I0,a 500 cycle tone is sent over the radio circuit where it is selected byband pass filter 40 and is amplified and rectified at 4| and operatescontrol relay 42. This relay is also slow acting on account of dashpot44. This may not be necessary in some cases, but the slow action ofrelay 42 is a means of assuring that the relay will not be operated bymomentaryv voice currents which might come through the filter 40 whenthe circuit is beingused for conversations. After the 500 cycle tone hasbeen on for a period Sucient to close the contacts 43 of relay 42,current will flow from battery 46 through the selector` switch arm 39 towhatever mechanism may be associated with the particular contact on theselector switch which has previously been dialed. For example, if theoperator at the control station dials .#1 a. single pulse of 2700 cycleswould go out from 8, be received at 28, and pass through 3| and 32 toactuate 33 to move the ratchet wheel RW one step so that contact 39comes to rest on contact #1 to close the circuit including 46 and bell50 at this point. Then when switch I0 of Figure 1 is operated to sendout a 500 cycle pulse long enough to close contact 43, the circuitthrough 50, 46 will be closed and the bell 50, which might be placed inthe living quarters of the attendant, for example, will ring. 'I'hisserves to call the attendant to the transmitter room in case ofnecessity. If the control operator wishes to correct the frequency ofthe transmitter, he may dial #4. This may nd out four 2700 cycle pulses,which will be received by 28. These pulses will advance the wheel RWforward four steps so that 39 would come to rest on contact #4 to closeat this point the circuit through source 46 and relay windings 5| and52. Now, if the operator presses switch I0, a 500 cycle note will besent out and 42 will be energized to close 43 and current from battery46 will energize relay windings 5| and 52 to close contacts and 56.Whenthe contacts 55 and 56 of these two relays close, they put thearmature of the motor 6| across thebattery 59 and the motor thenrevolves in one direction, such as, for example, the direction necessaryto raise the frequency of the transmitter. It should be noted that thefield of this motor, shown at 60, is permanently connected across thebattery 59.

If the operator at the control station had desired to lower thefrequency of the distant transmitter, he would have dialed #5 and thenthe operation of the on-oif relay 42 would have operated relays 53 and54. This would have closed contacts 51 and 58, putting the armature ofthe motor 6| across the battery 59 with the polarity reversed so thatthe motor would operate in the reverse direction and lower the frequencyof the transmitter.

It is obvious that the rotation of the motor 60, 6| may produce or causefrequency control in many ways, such as by operating a variable condenser or a variable inductance, etc.

'I'he particular arrangement used in the Hawaiian Islands, however, isthe long line control method and for the purpose of adjusting thefrequency, a trombone tube 61, connected by line 1 to 5', has beenconnected with a rack 66 and a pinion 65. Rotary motion of wheel 64 willdecrease or increase the electrical length of the long line 61 and raiseor lower the frequency of the power amplifier 5'. 'Ihe wheel 64 may bedriven by the motor in any manner. I have shown the wheel 64 as beingdriven by a belt, which in turn is driven by a pulley on the shaft 62 ofthe motor.

After an operation has been completed, as, for example, adjusting thefrequency of the transmitter, or calling the attendant by ringing thebell 50, the selector switch can be returned to the zero point bypressing the switch l2 of Figure 1. This sends out a continuous I2700cycle tone which energizes 33 continuously and holds relay contact 34closed. This enables the slow acting relay 36 to operate so that itnally pulls up latch or pawl 31 and allows a spring, not shown, toreturn the wheel RW and contact arm 39 to the zero point of the selectordevice. The selector switch is now in readiness for dialing any othercombination that may be desired.

Since the operation of the whole system depends upon the position of theselector switch contact arm 39, it would be highly undesirable for thisswitch to be operated by stray currents. Since the relay 33 must bequick acting, it is not possible to protectit against operation bymomentary voice currents with a dashpot as was done in the case of relay42. To avoid this difficulty, I propose to use a low pass filter 4 atthe control station, which cuts oil' everything above 2500 cycles, sothat no voice currents will exist at 2700 cycles, which might getthrough the band pass lter and operate the selector relay at the remotestation.

Incidentally, in connection with the operation of the slow relay 42, ofFigure 2, it should be pointed out that the relay is forced to closeslowly on account of the dashpot 44, but it must open quickly in orderthat it may return to its starting point. This switch is opened by thetension on the spring 45.

I have also shown means which will enable the operator, at one centraloflice', to ring over the raido circuit to the operator at a remotestation. ofllce, connected with the control station o'f Fig- 1 ure 1,sends ringing current over the line which looperates relay 9 of Figure 1to close .contact This relay is slow opening, so that as long as theWinding of relay 9 is energized by ringing current, the contacts willremain closed, thus sending a long continuous dash of 500 cyclemodulation outover the air.

At the remote station shown in Figure 2, the ratchet wheel RW will nothave been moved and it will be resting in a position such that 39 is onthe :ero point. 'Ihe long dash of 500 cycle modulation will energize thewinding of the slow acting relay 42 and close 43, thereby completing acircuit through 46 and energizing buzzer 48 with energy from battery 46.The contacts 49 under the magnetic action of 48 will then open and closeintermittently as in the case of an or dinary buzzer. 'I'he armature ofthis buzzer should be loaded mechanically so that it will vlbrate at aperiod of, say, 16 cycles per second, corresponding with the normalringing currents.

l:10 The output from this buzzer passes through transformer 41 onto themidpoints of the hybrid coil and thence to thecentral office at theremote station, where it signals the operator in the normal manner. Inorder to keep this buzzer current from'exciting the band pass filters 3|and 40l and thereby causing'false operation of the relays, I have showna coupling tube which isolates the filters from the ringing current.Itis obvious that the operator at the control station would have runglong enough for the relays to operate at the remote station, but therelays could be adjusted such that if the operator rings for three orfour seconds, the relays will voperate and at the same time will stillgive the desired protection against false operation by momentary voicecurrents.

At the remote point of Figure 2 four different operations may be carriedout, as described hereinbefore in connection with Figure 2.

1. The attendant can be called by sending out a single 2700 cycleimpulse and a. 500 cycle dash.

2. The circuit for raising the frequency of the transmitter 8' at theremote point may be prepared by sending out four 2700 cycle impulses toactuate the ratchet wheel and close the circuits through 5|, 52 so thatthe 500` cycle tone may be sent out to tune the transmitter.

3. The circuit for lowering the frequency of 6' may be completed byvsending out ve 2700 cycle impulses before the 500 cycle tuning impulsesare sent out.

4. Ringing current may be sent out to all remote stations from thecontrol station by sending from the control station ringing current toclose It is obvious that several other functions could be added. Forexample, the first operation of calling -the attendant could be arrangedfor starting up the transmitter, while operation two might be arrangedfor stopping the transmitter. A sixth operation could be used forincreasing the gain of the audio frequency energy supplied to themodulator, while a seventh operation could be arranged for decreasingthe gain of the audio frequency supplied to the modulator. Ihave notshownk these functions, as the means The ringing current from thecentral for carrying them out is perfectly obvious and it wouldunnecessarily complicate an understanding of the diagram if thesefunctions were shown a land line. I have shown such a modification inFigure 3. In this case, instead-of using two frequencies, I propose touse currents of two polarities, for example, if the switch 10 is thrownto make contact with contact 1|, and the dial 12 is operated, it willsend positive impulses over line 13, which will energize relays 14 and15 at the receiving end. These relays may both be quick acting. Relay 14must be quick acting. However,- these relays are polarized and thepositive impulses will not operate the tongue 16 of relay 15, but willoperate the tongue 11 of relay 14, thus causing the tongue to makecontact with contact 18. This connects battery 86 and energizes bothrelays 19 and 80, but since relay 19 is slow closing on account of thedashpot 8|, only relay will operate on the impulses from the dial 12.These impulses will notch up step by step selector relay ratchet wheel82 in accordance with the number of impulses dialed at the control endof the line. For example, if the operator dials 5 the selector switchratchet wheel 82 will rotate and 83 will make contact with point 45.Now, if the switch 10 is swung over to contact 84, a negative impulsewill be sent over the line. This negative impulse does not operate polarrelay 14, but it will operate polar relay 15, thereby passing currentfrom the battery 85, up through the selector switch and arm 83 to point5, then down to relay 81. When relay 81 is energized, it closes'contacts 88, 89 and 90, 9|, thus connecting the armature of motor 92across the battery 93, causing it to turn dial 94 in one direction. Onthe other hand, if the operator had dialed'#6, the same sequence ofevents would have operated relay 96, causing the motor to close contacts91 and 98, and turn the dial 94 in the opposite direction. Motion of thedial 94 may be utilized to tune signalling circuits or for otherpurposes. Whenl it is desired to return the selector switch to zero theswitch 10 will be thrown to contact 1| and a long impulse of positivecurrent will be sent out by operating switch |00, thereby closing polarrelay 14. and energizing slow relay 19, pulling down latch 13, andallowing the selector switch and ratchet wheel 82 to be pulled back toits zero position by a spring not shown.

The eld winding 95 of the motor at the remote point is energized by thesource 93. The positive and negative pulses dialed from the controlpoint dial 12 are supplied from batteries |0| and |02 respectively.

The system, of course, may also be used to control a transmitter, thatis, turn the transmitter on and off and tune the same from a remotepoint. For example, if the operator at the control station dials #9 ondial 12, relay 14 notches the selector switch 82 up nine steps so thatthe contact 83 comes to rest on point 9. Then when switch 10 is thrownto the right to complete a circuit through the negativel battery |02, 15is energized and the tongue 16 thereof is pulled against its springbiased to close its contact and can complete a circuit through battery85, selector switch point 9 to polar relay or winding |04. When thewinding |04 is energized the tongue |05 connected with the armature ofsaid relay is moved to the right to bear on contact v Cil |01. Thiscompletes a circuit from the power supply not shown to the transmitter||2, which in this manner may be rendered operative. The polar relayincluding the elements |05, |06 and |01 is so adjusted that the tongue|06 remains on contact |07 until the operator at the control point dials#8 after the switch 'l0 has again been moved to the left hand positionso that positive impulses of current passing over the control lineenergize the winding or relay 14 to close contact 'I8 and step theratchet wheel up to a point at which the contact 83 bears on point #8.This closes a circuit through winding |03 of the polar relay. Thearmature |06 is drawn to the left to bear on contact |05, thusdeenergizing the winding 0 of the power circuit contactor relay.Deenergizing winding ||0 permits contact |09 to open, thereby breakingthe energizing circuit to the transmitter, which becomes inoperative.The transmitter I2 may be replaced by a receiver. The receiver ortransmitter in ||2 may be tuned by the element to the desired frequency,as set out hereinbefore.

In conclusion, therefore, it may be noted that I have provided a novelmeans for checking the frequency of a plurality of transmittingstations. This checking operation may be done with a single monitoringcircuit, thus eliminating the necessity of separate frequency checkersat each transmitter. The present invention also provides means whicheliminates the necessity of keeping an attendant on duty at many of thestations since the apparatus at said stations may be controlled to theextent indicated above by my novel control scheme.

-Having thus described my invention and the operation thereof, what Iclaim is:

1. A monitoring system to be used with a signaling system including aplurality of transmitters each operating at different harmonicallyrelated frequencies comprising, signal receiving means of the heterodynetype including a radio frequency amplifier, a source of localoscillations and a demodulator, an oscillation generator of the constantfrequency type including'a crystal the frequency of which crystal isequal to lthe difference or to a sub-multiple of the difference betweenthe frequencies assigned to the wave transmitting stations, a connectionbetween said oscillation generator and the input of the demodulator ofsaid receiving means, an intermediate frequency amplifier coupled to theoutput of thedemodulator of said receiving means, and an indicatorcoupled to the output of said intermediate frequency amplier.

2. The method of monitoring a transmitting system including a pluralityof transmitting stations each oper-ating at different assignedfrequencies which are harmonically related which includes the steps of,producing high frequency local oscillations which may be beat with saidtransmitter frequencies to produce therewith beat notes, producingoscillations of constant frequency which frequency is equal to thedifference or to a sub-multiple of the difference betwee :he frequenciesassigned to the various transmitters of said system, producing harmoniesof said constant frequency oscillations which harmonics are equal to thefrequencies assigned to the different transmitters of saidsystem,combining said harmonics with said locally produced oscillations toproduce beat notes of intermediate frequency, and comparing said beatnotes of intermediate frequency with locally produced oscillations ofknown intermediate frequencies.

3. The method of monitoring a transmitting system including a pluralityof transmitters each operating on a different assigned frequency in- Ystant frequency to produce a beat note,l producing intermediatefrequency oscillations of va frequency of the order of.' the frequencyof said beat note, beating said intermediate frequency oscillations withsaid beat note to produce a second beat note, adjusting the frequency ofsaid locally produced oscillations of high frequency until Asaid secondbeat note is zero, beating energy from said transmitter to be monitoredwith said first named locally produced high frequency oscillations toproduce a third beat note, and comparing said third beat note with thefrequency of said intermediate frequency oscillations when said zerobeat note is obtained.

4. In a monitoring receiver to be used with with a signaling system inwhich a plurality of geographically spaced transmitters are operated atdifferent harmonically related frequencies, anl oscillator controlled bya crystal the irequency of which crystal is equal to the diiference orto a sub-multiple of the difference between the several harmonic-allyrelated frequencies of the transmitting station of said system, ademodulator of the superhetrodyne type coupled to said oscillatorcontrolled by said crystal, a wave receiving circuit which may receiveWaves from each of said transmitters, an intermediate frequencyoscillator coupled to said receiving means, and an indicator coupled tosaid intermediate frequency oscillator.

5. The method of checking the tune of a plurality of transmittersoperating in a system wherein each transmitter is assigned a frequency'which is harmonically related to the frequency of the other transmitterswhich consists in, producing high frequency oscillations of a constantfrequency equal to the frequency assigned to the transmiter to bechecked, producing other high frequency oscillations which differ fromsaid first produced oscillations of constant frequency by a beat note ofintermediate frequency, beating said oscillations to produce said beatnote of said intermediate frequency, producing oscillations of a knownintermediate frequency, beating said last produced oscillations of knownintermediate frequency with said beat note, and

varying the frequency of said Second produced high frequencyoscillations until zero beat note is obtained .between said intermediatefrequency beat note and said produced oscillations of known intermediatefrequency, resonating energy characteristic of the energy transmittedfrom the transmitter to be checked, beating said resonated of whichcrystal is equal to the difference or to a sub-multiple of the dierencebetween the harmonically related frequencies ofthe transmitting stationsof said system, a harmonic generator coupled to said oscillator, 'signalreceiving means, amplifyingmeans coupled to said signal receiving means,a demodulator coupled lto said amplifying means and to said harmonicgenerator, a source of high frequency oscillations couplied to saiddemodulator, an intermediate frequency amplifier coupled to saiddemodulator, an intermediate frequency detector coupled to saidintermediate frequency amplifier, indicating means coupled tov saidintermediate frequency detector, and a source'of intermediate frequencyoscillations of variable frequency coupled to said intermediatefrequency detector.

7. The method of monitoring a plurality of transmitter stations eachoperating at different harmonically related frequencies which comprises,producing oscillations of a constant frequency which are equal to thedifference or to a sub-multiple of the difference between thefrequencies of theseveral transmitter stations, producing harmonics'ofsaid frequency, combining said harmonics with energy from saidtransmitters to produce a beat note, and comparing said beat note withthe beat note produced between other high frequency oscillations andsaid har#- monic frequencies.

8. The method of monitoring a transmitting system including a pluralityof transmitters each operating at different assigned harmonically re.-lated frequencies, which includes the steps of, producing oscillationswhich are equal to the differ ence or to a sub-multiple of the diierencebetween the assigned harmonically related frequencies, producingharmonics of said oscillations equal to the assigned frequencies,combining said harmonics with energy from said transmitters to producebeat notes, and comparing saidbeat notes with beat notes produced bybeating other local oscll.

lations with said harmonics.

9. The method of monitoring a transmitting system comprising a pluralityof ,transmitters each operating at different assigned frequencies whichare harmonically related including the steps of, producing oscillationsof a constant frequency equal to the difference-or a sub-multiple of thedifference between the assigned frequencies, producing harmonics of saidoscillations, combining said harmonics with signal energy from saidtransmitters, demodulating said combined energy to produce beat noteenergy `of an intermediate frequency, producing other localoscillations, beating said other local oscillations with said harmonicto produce other beat notes, and comparing the frequency of said beatnotes. Y

10. The method of monitoring a transmitting system comprising aplurality of transmitting stations each operating at different assignedfrequencies differing by a constant difference frequency which includesthe steps of, producinglocal oscillations of high frequency, producingconstant frequency oscillations equal to the difference or asub-multiple of the difference between the frequencies assigned to saidseveral transmitting stations, producing harmonics of said constantfrequency oscillations, beating said high frequency oscillations withsaid harmonic frequencies to obtain beat notes, combining said highfrequency oscillations with signal energy,

